Hydraulic Functions
Hydraulic Functions
NOTE: Numbers in parentheses ( ) indicate a component or tool in the associated illustration.
CONTROL SECTION
The hydraulic control section basically comprises the pressure chambers (power chambers) for servo assistance for lee (1a) and right lock (1b), the steering nut (25) with the control-bar and the spool valve (4).
A high-pressure seal (1c) is located in a groove in the steering housing (1) which seals off the moving power piston (11) from the pressure chamber (1a or 1b).
The spool valve (4) moves in a bore transverse to the power piston (11 ) in the steering housing and has a precise fit.
The spool valve is sealed off from the outside and secured by a screw cap (8).
The control-bar engages in a groove in the spool valve (4) and forms a unit with the steering nut (25)
The spool valve and the steering nut together with the control-bar are precision-manufactured parts and matched to each other.
CENTER POSITION
The cylindrical spool valve (4) has leading edges, which are likewise located in the housing bore in order to achieve a selected oil control by disp1acing the servo spool valve.
When no steering forces are acting, the spool valve is held in the center position by a centering spring (12).
In this position the control cross-sections A = "from the power steering pump" and B = "to the power steering pump" are open as are the two connections to the pressure chambers (1a, 1b) so that the oil circuit is not restricted.
LEFT OR RIGHT LOCK
(Left Lock Shown)
Steering force assistance is initiated when the control-bar of the steering nut (25) moves the spool valve against the centering spring. In this way pressure oil is introduced into the pressure chamber (1a) via the leading edge (b) of the spool valve.
At the same time the circu1ating oil is forced back to the other side of the piston via the open leading edge (d) in the supply reservoir (B) by disp1acing the power piston.
REACTION PROCESS
(Example On Left Lock)
The reaction process is initiated by introducing pressure oil into the reaction chamber via a throttling port (arrow).
Provided the reaction piston (34a) is supported in the housing via the compression spring (35b), the pressure oil in the reaction chamber causes a restoring force to act on the spool valve (4). Within the normal driving range, i.e. with a torque at the steering wheel of greater than 2 Nm, the restoring forces in the reaction chamber are proportional to the oil pressure at the power piston and thus to the steering moment at the front wheels.
This reaction effect causes the moment introduced at the steering wheel to correspond to the pressure assistance.
The greater the torque acting or, the front wheels, the greater the torque to he applied to the steering wheel. The level of oil pressure assistance does not therefore depend on the turn angle of the steering wheel but merely on the torque acting on the front wheels.
FUNCTION DIAGRAM
The operating range of the steering gear is shown graphically in the diagram. The torque on the steering wheel is transmitted by the torque On the pitman arm shaft and me oil pressure. The diagram shows how, as torque increases, the increase in servo assistance is directly proportional (up to approx. 5 Nm).
The size of the reaction surface (diameter of reaction piston) determines the level of force which is assigned to a particu1ar oil pressure.
MANUAL EFFORT LIMITATION
The simple linear reaction with manual effort limitation limits the range of proportionality between the torque on the steering wheel and the torque on the pitman arm shah and has a constant curve thereafter.
At the same time the reaction piston (4b) is supported from a particu1ar oil pressure on the stop (4a) of the spool valve (e.g. circlip) so that the forces acting on the spool valve and reaction piston from the oil pressure become internal forces. Only the constant spring preload can be detected externally. The oil pressure assistance increases from this point up to maximum pump pressure of up to 125 bar, without the torque on the steering wheel exceeding 5 Nm. This avoids unacceptably high steering forces, e.g. during parking.